1. Field of the Invention
The present invention relates a DA (Digital-to-Analog) converter for converting a digital signal into an analog signal, an AD converter including the DA converter, and a semiconductor device such as an imaging device, etc. The present invention relates to, for example, a mechanism of a DA conversion using a current-source cell matrix having an array of cells arranged in a matrix.
2. Description of the Related Art
DA converters for converting a digital signal into an analog signal are used in various ways in electronic apparatuses, for example, cameras or cell phones, or in semiconductor devices.
For example, a semiconductor device for detecting a physical-quantity distribution, in which a plurality of unit components (for example, pixels) having sensitivity to electromagnetic waves input from the outside, such as light, radiation rays, etc., are arranged in line or in a matrix, are used in various fields.
For example, in the field of video apparatuses, solid-state imaging devices of a CCD (Charge Coupled Device) type, a MOS (Metal Oxide Semiconductor) type, and a CMOS (Complementary Metal-Oxide Semiconductor) type, which detect light (an example of an electromagnetic wave) among physical quantities, are used. These imaging devices read out a physical-quantity distribution converted into an electronic signal by a unit component (a pixel in the case of a solid-state imaging device) as an electronic signal. Here, “solid-state” means that the device is made of a semiconductor.
Also, solid-state imaging devices include an amplified solid-state imaging device including a pixel having the configuration of an amplified solid-state imaging element (also called APS; Active Pixel Sensor/gain cell), which has a transistor for amplification in a pixel-signal generation section for generating a pixel signal in accordance with signal charge generated by a charge-generation section. For example, most of CMOS imaging devices have such a configuration.
In such an amplified solid-state imaging device, in order to read out a pixel signal to the outside, address control is performed on a pixel section in which a plurality of unit pixels are arranged, and the signal is arbitrarily selected to be read out from individual unit pixel. That is to say, an amplified solid-state imaging device is one example of solid-state imaging devices of an address control type.
For example, in an amplified solid-state imaging device, which is a kind of X-Y addressing-type solid-state imaging device, pixels are composed of active devices having a MOS structure, etc., (MOS transistors) in order to have an amplification function in pixels themselves. Specifically, signal electric charge (photo electrons) stored in a photodiode, which is a photoelectric transfer device, is amplified by the active device, and is read out as image information.
In an X-Y addressing-type solid-state imaging device, for example, a large number of pixel transistors are arranged in a two-dimensional matrix to constitute a pixel area. The storage of signal electric charge corresponding to incident light is started for each line (row) or for each pixel. The signal of electric current or voltage based on the stored signal electric charge is read out in sequence from each pixel by addressing. Here, in a MOS (including CMOS) type, a method, in which pixels of one row is accessed simultaneously, the pixel signals are read out from the pixel section for each row, and are output to the outside, is often used. Also, a method, in which analog pixel signals read out from the pixel section are converted into digital signals using an analog-to-digital converter (AD converter) and then are output to the outside, is sometimes employed (for example, Japanese Unexamined Patent Application Publication Nos. 2000-152082 and 2002-232291).
As described in Japanese Unexamined Patent Application Publication Nos. 2000-152082 and 2002-232291, there are various kinds of AD conversion methods in terms of a circuit size, a processing speed, a resolution, etc. Among them, there is an AD conversion method of a so-called single-slope integration type or a ramp-signal comparison type, in which a comparison is made between an analog unit signal and a reference signal changing uniformly for the conversion into a digital signal, count processing is performed concurrently with the comparison processing, and the digital signal of the unit signal is obtained on the basis of the count value at the time of completion of the comparison processing. Thus, in order to generate the reference signal changing uniformly, a DA converter is sometimes used.